Specific features of shale compaction in the Azerbaijan sector of the South Caspian Basin

Abstract

The South Caspian Basin (SCB) covering a small area experienced intensive subsidence in the lower Pliocene and was isolated from the world ocean. The structures of the Baku Archipelago have one of the world's most complex geological conditions for drilling, with rocks in the near well-bore area squeezed into the wellbore (the so-called well filling) when drilling through shales in the productive series, which account for as much as 90% of the Archipelago. Well filling leads to problems and accidents and triggers drilling mud blowouts and water/gas kicks, which often results in well abandonment.The primary cause for well filling is abnormally high pore pressures in the cross-section of the structures, with gradients close to geostatic pressure gradients; such pressures are typical of thick shale strata. A study of shales found that shale porosity remained exceptionally high at great depths due to delays in shale compaction, the shortage of de-stressed zones and high rates of sediment subsidence.Based on the core studies and measurements while drilling, various parameters were analyzed, including the density and porosity of shale samples taken in different years while drilling on the Baku Archipelago, in particular on the Umid structure. The compaction curve was plotted, and a potential zone of abnormally high pore pressures was delineated.